A turbofan gas turbine engine 10, as shown schematically in FIG. 1, comprises in axial flow series an inlet 12, a fan section 14, a compressor section 16, a combustion section 18, a turbine section 20 and an exhaust 22. The fan section 14 comprises a fan rotor 24 carrying a plurality of equi-angularly-spaced radially outwardly extending fan blades 26. A fan casing 28 that defines a fan duct 30 surrounds the fan blades 26 and the fan duct 30 has an outlet 32. The fan casing 28 is supported from a core engine casing 34 by a plurality of radially extending fan outlet guide vanes 36.
The turbine section 20 comprises one or more turbine stages to drive the compressor section 18 via one or more shafts (not shown). The turbine section 20 also comprises one or more turbine stages to drive the fan rotor 24 of the fan section 14 via a shaft (not shown).
One known wide chord fan blade is disclosed in US2004/0018091 to the present applicant and is depicted in FIGS. 2 and 3. The blade 26 comprises a root portion 40 and an aerofoil portion 42. The root portion 40 comprises a dovetail root, a firtree root, or other suitably shaped root for fitting in a correspondingly shaped slot in the fan rotor, or for mounting to a disk to form a blisk by linear friction welding or other appropriate method. The aerofoil portion 42 has a leading edge 44, a trailing edge 46 and a tip 48. The aerofoil portion 42 comprises a concave wall 50, which extends from the leading edge 44 to the trailing edge 46, and a convex wall 52 that extends from the leading edge 44 to the trailing edge 46. The concave and convex walls 50 and 52 respectively comprise a metal for example a titanium alloy. The aerofoil portion 42 has an interior surface 54 and at least a portion, preferably the whole, of the hollow interior 54 of the aerofoil portion 42 is filled with a vibration damping system 56.
The damping material 56 is a relatively low shear modulus material having viscoelasticity. Viscoelasticity is a property of a solid or liquid which when deformed exhibits both viscous and elastic behaviour through the simultaneous dissipation and storage of mechanical energy. Suitable materials comprise a polymer blend, a structural epoxy resin and liquid crystal siloxane polymer.
One particular and preferred polymer blend comprises, per 100 grams: 62.6% Bisphenol A-Epochlorohydrin (Epophen resin EL5 available from Borden Chemicals, UK); 17.2 grams Amine hardener (Laromin C260 available from Bayer, Germany); 20.2 grams of branched polyurethane (Desmocap 11 available from Bayer, Germany). This polymer blend is then mixed in a mass ratio of 1:1 with a structural epoxy resin, preferably Bisphenol A-Epochlorohydrin mixed with an amine-terminated polymer (e.g. Adhesive 2216 available from 3M).
A fan is susceptible to Foreign Object Damage, or FOD. Composite blades are not as robust as metal blades but offer advantages in terms of reduced mass. Where a hollow blade is provided there is a risk that the blade may burst when impacted by a large object. The use of a viscoelastic filler or core offers damping but also offers a secondary advantage in that the sides of the blade are held together to resist bursting, particularly busting at the trailing edge tip. Blade robustness may be improved through the provision of an internal warren truss arrangement as shown in FIG. 4 where metal girders 60 extend between the concave face 50 and convex face 52 of the aerofoil. The viscoelastic damping material extends around the girders 60
The girders inhibit bursting of the blade upon impact by foreign objects but provide a pathway for the transmittal of vibrational loads through the damping material which can render such damping material obsolete.